The use of Sallen-Key type filters is well-known for use in various applications. A typical Sallen-Key filter has four key elements: A serially connected resistor pair, an output amplifier, and two capacitors. The resistor pair is connected in series at an input of the amplifier. One of the capacitors is a feedback capacitor, connected between the output of the capacitor and a node between the resistor pair. The other capacitor is connected between the amplifier input and ground. Often, the circuit is symmetric, in a differential configuration, so that a second resistor pair is connected in series to a parallel input of the output amplifier, with a corresponding second feedback capacitor in the parallel branch. It is also known to employ an input buffer before the actual filter, in order to provide a high input resistance to the circuit and at the same time to separate the filter resistors from the output resistance of the previous circuits. An advantage of this filter type is that it may be easily implemented on a chip, and it is also suitable for multistage filter assemblies, such as those commonly used in wireless applications.
Recently, a number of known electronic devices have entered the market in a portable, battery-operated version. At the same time, many electronic devices have been equipped with wireless features, for example for communicating data between an outdoor sensor and an indoor receiver unit. All these applications require circuits which should have low operating voltage and low power consumption. Operating voltages as low as 1.5 V are required, so that even a single battery may power the device. Low power consumption is required for the very same reason.
The requirements for these low-power devices conflict with some of the traditional principles of circuit design. For example, in an amplifier circuit with a separate input buffer stage it is common to use multiple levels of transistors. Integrated circuit (IC) chip transistors are often FETs or MOSFETs, which each need an operating voltage of about 1.2–1.3 V. This means that two transistors in series between the power supply voltage Vdd and ground require at least 2.4–2.6 V for operation.
A further requirement of IC is a capability for process compensation. This may be realized by providing variable resistors or capacitors in the circuit. Since the Sallen-Key filter is tunable, further variable elements are also needed to perform the tuning of the circuit.
U.S. Pat. No. 5,418,492 to Wang et al. discloses a fully differential Sallen-Key type filter, which contains a unity gain follower instead of an operational amplifier (op-amp). The transistors of the filter are NPN devices, and the unity gain follower contains two transistors in series between Vdd and ground, accordingly the operating voltage must be 3 V or above. Process compensation is not addressed.
U.S. Pat. No. 6,097,244 to Chen discloses a linear and tunable filter, designed for a 3 V supply voltage. The filter is provided with a voltage controlled oscillator (VCO), which controls variable capacitors with a successive approximation register (SAR), and thereby adjusts the capacitance of the circuit. The capacitance in the VCO is also adjusted. Further, variable resistors are also provided, so both capacitance and resistance are adjusted in an interdependent manner.
U.S. Pat. No. 6,407,627 to Martin discloses a tunable Sallen-Key filter, which includes an adjustable resistor ladder. The filter is tuned by switching resistors in the ladder.